N3OX 3m Flex Vertical, a Short, Frequency Agile Vertical

Introduction

When I started out in ham radio I started with a random wire antenna that was pretty uniformly mediocre on all bands. It just worked okay. The first
antennas I experimented with in comparison were simple types like resonant half-wavelength dipoles and ground plane verticals, and they always
seemed to work amazingly well by comparison. I learned quickly that it was very difficult to beat a half wavelength dipole. Because of this
, there was a time some years ago when I was one of those people who would give antenna
advice along the lines of "go full size or go home." I still think there's some merit to that when possible, but I've also gained a better appreciation
that by choice or by necessity, many people have a need for small antennas that are solid performers.

But the people who need small antennas have a very hard road to travel. There are a lot of terrible designs
out there. There are dozens of them that do nothing besides drive current on the coax shield.
Those types will often claim some kind of "new physics," which always somehow seems to hinge on you leaving
a balun off the antenna! No one ever seems to put a current meter on the feedline either. A one foot antenna with 20 feet of radiating coax is is
not a one foot antenna, and it often means part of the "antenna" is strung inside a noisy house.
Not everyone claims that their antenna runs on a new physical phenomenon. But many people who accept established EM theory but
don't actually apply it very often will claim that their novel contribution
to the antenna is the main reason why it works well.

From time to time, someone who's selling something is
clearly trying to sucker us out of our
hard-earned money. Much of the time, though, people are just excited about their homemade creation and are happy with how it works on the air,
and they simply seem to think WE just don't need to know more than
that. They're confident that it works, it makes contacts, so
just TRUST THEM and BUILD IT and TRY IT, right? Who knows, maybe there are
really advantages to their designs. But it's natural and
correct for the rest of us to
be skeptical unless we see some kind of satisfying comparison against a more familiar antenna. It doesn't take a lot of time reading
antenna reviews to realize that people are happy with just about anything. This is because ham radio is great, but the fact that someone
is happy with an antenna of unknown performance is not very helpful to pick an antenna out of the many options. We all know that some people
are happy with antennas we'd find unusable.

This project is designed partially to serve
as a counterpoint to the flood of weird designs out there and the lack of measurements. I assure you up front that this antenna's operation is completely explained using well-tested
physical principles, and can be easily modeled in common modeling software. From an electromagnetic
standpoint the antenna I built is just totally dumb and boring and easy to understand, and I think there is great value in that. At the same time,
I think I've put something of a novel twist on this antenna that seems to have real practical advantages. It's my
creation, despite the old-hat electrical design. I don't need to confuse you to convince you I made something new. If the design is straightforward,
and I convince you that there's nothing too critical about it, you'll be confident in adapting it to different materials, bands, and sizes.

In addition to describing a small antenna
that works well, I want to provide reproducible evidence that it
works well and
demonstrate
some meaningful
measurements done with equipment you might already have.
It's easy to be over-awed by the equipment and procedure that goes into
a good,
controlled
experiment like
N6LF's
series of ground radial experiments. Someday I'd like to have a
vector network analyzer to do this sort of thing. It would speed things
along a bit. But a lot of what's important to a good study like N6LF's
is the procedure, and good procedure only requires time and practice.
The measurements I did are the kind of thing that anyone can do with a
litle setup
if they'd like to satisfy their skepticism about a new design.
With a little care (and healthy self-skepticism so you don't fool yourself), they're
vastly more precise than trying to assess an antenna over the air.
If you are
set up to do
measurements, it
means you can
modify
and redesign the
antenna and still remain confident that it's working well.

What to build?

So, fine. I've decided that I need a pretty small antenna, and that I want some kind of straightforward, reliable design. But WHAT can I build? There is ample good information
out there on how to make good small antennas, in books and even on the web. There are pages that have fairly general advice with solid engineering
analysis on how to build good small loaded antennas, like some of the pages on
W8JI's Website. There is a fair amount of specific construction information for certain types of antennas, like the
very popular magnetic loop out there. AA5TB's Website
is a great jumping off point for that.

If you stick to technically accurate sites and books and study the general principles of electrically small antennas, you'll know that there is a
fundamental tradeoff among three things:

Efficiency

Bandwidth

Physical Size

You will come to appreciate that you need to maximize the
radiation resistance and minimize the loss
resistances in order to keep small antenna efficiency and
bandwidth as large as possible. These are very important
general principles, and for any given type of antenna, it's
clear how to do that. For example, if you build a magnetic
loop, you should build it as big as is
practical, you should solder or weld the joints, and you
should use a very high quality transmitting capacitor. You can
do pretty well that way.

But there's a weird problem
that's been nagging at me. Among homebrewers, the magnetic
loop seems to be one of the the most popular small antenna
projects that are based on solid physical principles. I've got
one. But it most definitely does not maximize
radiation resistance. Not even close. The enormous currents
in typical magnetic loops, 100+ amps for the brave souls who
try them at kilowatt power levels on the lower bands, are due
to the fact that the radiation resistance of the magnetic loop
is really tiny. The current going "up" on one side of the loop
at one instant in the RF drive cycle is nearly totally
cancelled by the current going "down" in the diametrically
opposite point. The currents cancel perfectly in the broadside
directions of the loop, giving deep nulls. There's a tiny
phase delay from one side of the loop to the other for
radiation leaving in the plane of the loop, and that's what
does all the radiation. But you need huge currents to radiate
even a small amount of power.

The practical upshot of
this is that you need to make the antenna of very low loss
stuff, like copper pipe, and you need to use a very low
resistance capacitor to tune it to resonance. You also need to
use a very, very high voltage capacitor for just 100W, and if
you want to run higher power, you soon have no choice besides
buying a $200 or more vacuum variable capacitor. The
operational bandwidth of a magnetic loop is quite tiny at one
tuning setting, to the point where some people have built loops
for 160m or 80m that barely pass a single SSB signal. But they
remain very popular among the homebrew crowd. Why do people
put up with low power handling, field cancellation,
expensive materials, an expensive capacitor, and the need to
constantly retune if you QSY even a few kHz on a low HF band?

I think one of the reasons is quite simple: it has continuous
tuning that can easily be made remote controlled. Who
really cares if you have a 3kHz 2:1 VSWR bandwidth on 40m if
that bandwidth can move anywhere in the 40m band? Who cares if
the tuning all changes down 100kHz when it rains or snows?
The tunability completely removes the bandwidth restriction and
lets you compensate for the environment. Sure, you can build a
fixed-tuned dipole or vertical type antenna with 50kHz of
bandwidth on 80m, but if you always have to run outside with
your antenna analyzer any time the antenna gets damp to re-tune
it, you're going to go nuts. Continuous tuning can also be
achieved by a home station antenna that's based around a
screwdriver-type tunable inductor. But a screwdriver antenna
is a low-volume custom manufactured part that's generally more
expensive than a vacuum variable. It's even worse if you want
independence from ground radials and want to build a dipole
with TWO coils. Not to mention that it might not feel like so
much of a homebrew project if you just order a couple of
beautifully made Scorpion screwdriver
antennas. Roller inductors will probably move you around a
couple bands, and certainly let you move around inside a band,
but they seem a little hard to find and I don't want to try to
keep a rolling contact working outdoors. Furthermore, it seems
quite difficult to build a reliable adjustable inductor at
home even for low power, and people who want to build low power magnetic loops have several
good options for homebrew capacitors.

I
think these reasons may explain why a magnetic loop is so
popular, even though it has some significant disadvantages vs.
other types of electrically small antenna. I wanted to build
something that would have higher radiation resistance while
being more convenient like a small transmitting loop.

So what design goals did I have in mind when I came up with
this antenna?

High radiation resistance for the size,
suggesting a dipole-type antenna with heavy end loading.

Some measure
of continuous tuning, in particular I wanted enough to
cover the entire 40m band including some extra for WX drift.

The continuous tuning should be easily remote controlled,
as easy or easier than a magnetic loop.

No expensive
special components. I'm fond of designing antennas that you
could build from stuff you found at Home Depot.

Cover a
good number of bands.

Go small, but not absurdly small, to
keep performance very high and demonstrate that "full size
or go home" is not reasonable advice.

I didn't really think about power handling at the outset,
because I've been a hundred-watt-ham until very recently, but
I'm sure this should handle legal limit without too much
trouble (maybe not 1500W RTTY with the PVC coil form). It's
been tested to 600W with no problems. The antenna I finally
built and tested met most of the design goals. However, for
the sake of expediting the installation and doing the testing
testing, I gave up on building a dipole and instead built a
ground-mounted vertical. I don't have a good way to support a
40m dipole but I could easily put up a quarter wave 40m
vertical as a reference antenna. I have radials down already,
and in my personal opinion, they're a big asset to stealth
operation from a home you own. The results here, however,
should apply just fine to the dipole version. My antenna is
a three-band antenna. 40m, 30m and 20m, and is at worst 2dB
down from a quarter wave vertical. Unfortunately band
switching is not fully remote-controlled, and does require
changing coil taps to change bands. It is easily remotely
retuned to cover all of each band. Enough setup, let's get on
with it.

N3OX Three Meter Flex Vertical - Design and
Construction

Since I planned to start
with a dipole, the antenna wasn't necessarily intended to be a
homage to Jerry Sevick, W2FMI, but it kind of turned out that
way. I've always wanted to try something like his six foot 40m vertical,
but never got around to it. In the end this antenna is pretty similar, though it
actually is about ten feet (3m) tall (0.075λ on 40m) because
the capacitance hat is a tall pyramid shape. The model
schematic from EZNEC on the right below shows the parts of the
antenna that are electrically conducting. My version, depicted
on the left, has a 76 inch vertical part made of tubing from DX
Engineering and a wire pyramid capacitance hat supported on
four 3/16" diameter 4 foot long
fiberglass rods from Max Gain Systems.

The finished antenna (left) and a schematic of the simple model geometry (right).

The antenna is loaded to resonance by the
large capacitance hat and a conventional high Q inductor wound
on a machined PVC coil form pictured below. The coil is mounted to
a short piece
of fiberglass tube which also provides a strong joint betwen the two
tubing sections about 2/3rds of the way up from the base. I realize
the kind of fancy construction of these parts is not in keeping with the claim I made about "simple
materials," but I intended this antenna to be a keeper and I decided to spend
some money and spend some time in the machine shop with the
lathe. A nice way to make coils without that facility can be
found on AD5X's Website under "Build your own air-wound coils and
coil supports." The strong fiberglass insulator is overkill
given the likely forces on this short antenna. You could make
this out of EMT tubing and PVC pipe, or all PVC with a wire
radiator, and coils like AD5X suggests.

IMPORTANT: You might consider the use of several wires in
parallel, like a "cage" radiator if
you use a PVC and wire construction. I ran some more recent models and
thinner wire or tubing than my 1.25" tubing requires a larger loading
coil for resonance. A larger loading coil means more loss resistance
and a narrower operating bandwidth.

If you
can find thin
fiberglass driveway markers or
suitable fishing rod parts you might really be able to make a version of locally sourced parts.
The coil has too many turns for 40m on the final antenna, but has too few turns for
60m. I can get down to 5.6MHz if I tap at the top, so probably
17 or 18 turns 10AWG 4TPI on a 3.5" PVC pipe or slightly
compressing a coil that can slide would get you on 60m without
giving up 20m. The coil should actually be closer to the
capacitance hat for optimum radiation resistance. Its location
was set in a prior design iteration but I didn't move it
hatward because the modeled improvement didn't seem worth
rebuilding that part.

On 40m, the antenna's 2:1 VSWR bandwidth is
about 73kHz. But that 73kHz bandwidth can be tuned all the way
up and down the band. Four ropes are connected to the midpoint
of each of the horizonatal wires in the capacitance hat, and
when those four ropes are pulled by a common rope that goes
down inside the vertical tubing, the fiberglass support rods
bend, and the hat collapses in on itself. The slightly smaller
hat raises the resonant frequency of the antenna. The photos
below show it tuned to different frequencies. I don't recall
the full tuning range, but it's something like 6.95MHz to maybe
as high as 8MHz when the fiberglass rod tips are nearly
touching. Since this antenna handles high power and can be
tweaked easily for an ultra-low SWR at every frequency it would
be a good choice to pair with a solid-state amplifier. No high
power tuner needed.

The capacitance hat when the antenna is tuned to the low end of the range (left) and higher in the range (middle).

Another shot of the partially collapsed capacitance hat is shown below.
You might note that the Q of the antenna is a bit higher at the higher end of the band because the capacitance is less and
the inductance stayed the same, and I can just detect that in the measured 2:1 VSWR bandwidth. I think I get closer to 75kHz at the bottom of 40m and
72kHz at the top. The whole antenna is pretty low Q for the size because of the big hat, so it can efficiently cover a wide bandwidth. Among other
things, this means that the tuning isn't touchy.

Partially collapsed capacitance hat from the side

The four ropes connected to the capacitance hat top wires enter
the hub as shown below left and are tied together into a single knot. Originally a larger single rope threaded out the bottom of the vertical pulled on this knot to
pull the four tuning ropes into the hub. I did some testing which I will get into later and decided I was worried about a wet rope shunting across
the high Q loading coil, so I replaced the first four feet of that rope with another fiberglass rod with some cable ties attached to the ends.
The fiberglass won't absorb water. I have no direct evidence that I was having "wet rope" problems, but we'll discuss that more later. The fiberglass
pull rod (below right) across the inside of the coil gap makes me feel better.

The capacitance hat spoke hub with ropes (left) and the end of the fiberglass pull rod that reaches from the ropes to down to below the coil (right).

Below is a short video of the hat in operation.

When I originally envisioned this antenna, I figured I'd use some kind of motor reel to wind up the rope, and I think that could be a cool
thing allowing computer controlled tuning. But for testing, I set up the simplest possible remote control. I ran the rope out the bottom
of the vertical (running over the edge of the DX Engineering Resin Support Block insulator, not the sharp edge of the aluminum), and then just ran
the rope back to the shack through a snug fitting hole in my cable entrance panel. Sweet and simple. Shown below is the pull rope and some added
elastic cord tied to the base of the vertical and tied to the rope. This gives a better pull-back for the rope lying in the grass. The fiberglass rods have plenty of pull
if you don't use a very long "remote rope" but the elastic helps get the hat back to maximum expansion when the "remote rope" is long. To hold
the tuning fixed once I've tuned it, I just hitch the rope to a support arm under one of the radios on my desk. No kidding. It's like the
exact polar opposite of my last remote control project, but sometimes a simple
system is the best system for the job. Right now I don't have a remote tap changer, and I just go outside to change bands,
but something with relays would be easy to cook up. I've mostly been using the antenna on 40m where it's electrically smallest, because that's
really the main point of these experiments. The other bands should work even better than 40m.

Tuning rope with elastic pull-back assist.

The initial model runs using perfect ground suggested a radiation resistance around 5.4 ohms, and it seems that my coil and my radial field,
27 radials out to the edges of an approximately 50 by 50 foot area, only add a little to that number. The antenna base impedance is about 6 ohms
at resonance. I matched it to the coax using a shunt inductor across the feedpoint, with about 0.43μH needed.

Okay, now it's all put together and shows a low SWR and tunes just like I planned. Great! I got a 59 report from V55V/P on 40m the first night.
That's Namibia on a ten foot 40m vertical! What more can I say? We're all done here. Go build it... nah, just kidding. Let's MEASURE stuff.
But before we do that, let me say that getting some good reports from some Europeans and Namibia on this thing was actually really exciting. It
is clearly a good antenna, and I could tell that before I measured anything. I understand this. But one
question I have to ask myself is this: should I leave this installed for a while, or put my new 30/40m trap vertical back up, or my 60 foot vertical
that will work 80m adn 160m too. And if I use this for a while, how much of a disadvantage will it be if a new country comes up on 40m CW? 1dB?
6dB? What? Well there's an easy way to determine that. Let's look at some data

Measurements and Model Comparisons

The easiest and first comparison is impedance and VSWR vs. the model. Here I'm presenting the measurements and models including the parallel transformation
with the shunt matching coil. Since I predicted a radiation resistance of 5.4 ohms and measured 6 ohms I
added 0.6 ohms to the loading coil in the model, and I calculated these model data using MININEC type ground which is lossless in the near field. The model with just 6 ohms total
resistance and a 0.43μH shunt seems to reproduce the measured impedance data well. We won't trust the 5.4 ohm
radiation resistance number to calculate the efficiency, however. That implies just 0.5dB loss with respect to a quarter wave vertical and
I'll show later that -0.5dB vs. a 1/4λ seems too optimistic. If I put in "realistic" radials into the model, and use Sommerfeld-Norton
"High Accuracy" ground, it doesn't properly predict the impedance, VSWR curves, etc.

Measured and modeled resistance and reactance with the shunt coil installed.

There are many tricky issues in the model/measurement comparison. The radiation resistance from a short vertical has an important contribution
from its image in the ground. Over perfectly conducting ground (or MININEC, which is also perfectly conducting in the nearfield) the radiation
resistance of a short monopole is doubled vs. its free space value (here, 5.4 ohms perfect ground, 2.7 ohms free space). The data and models suggest that it's not quite doubled over real earth, but
the Sommerfeld Norton models for realistic ground parameters don't agree with reality in terms of losses and base impedance.
The details of this need their own article, however. We will not be relying on model/measurement comparisons to infer the antenna's efficiency,
though the comparisons here do suggest a fairly low loss antenna.

The measured and modeled SWR curve is given below for the same measurement data. This is probably a more useful curve for would-be builders. The agreement is good, and I consistently get a 2:1 VSWR band
width between 72kHz and 75kHz at any one tuning setting. This is nice, because it doesn't require particularly frequent retuning, and it can rain
and blow around the antenna a bit without knocking the SWR for a loop.

Measured and modeled SWR curve with the shunt coil installed.

The measured impedance data already suggest good agreement with the model. However, there is a lot of value in measuring
exactly what you want to know, which in this case, is a gain or efficiency comparison against a reasonable reference antenna.
The natural choice for a comparison here was a simple 1/4λ vertical. I made one from 18AWG wire wound on the outside of
a Spiderbeam fiberglass pole, which was easy to take up and down. The reference antenna was resonant around 7300kHz not far
from the testing frequency. It had a base impedance of about 26 ohms at resonance, which I matched to nearly 50 ohms using a series
section transformer identical to the one on the bottom of
this page. Both antennas need to be matched to 50
ohms for this technique to work easily, but I already did that anyway.

To simplify the necessary equipment and speed the data collection, I chose to measure the difference between the antennas on reception by
using a small battery powered oscillator a certain distance away from the antennas under test, as pictured above.
I used a battery powered TTL clock oscillator, pictured below, at the feedpoint of a six foot high vertical dipole.
There's some other circuitry, there but it's not needed. I used the fourth harmonic of an 1843.2kHz oscillator to test the antenna just above the
40m band. An adjustable attenuator on the input of the radio is useful to set the level of received signal such that there is no distortion in any receiver
stage. However, a large value of attenuation should be used to terminate the feedline at
the receiver. It is important that the SWR on the line is low when the antenna is the "source" and the line and receiver are the "load"
as in this test. The RX input impedance is not necessarily 50 ohms, and mismatch to the line will cause extra losses in the received signal,
and uncertainty in signal measurements when two different lines are used (as in this case). In some cases this attenuator is not
needed but it's good insurance.

In my case about 40dB of attenuation gave a good signal to noise ratio without any gain compression or
distortion in the receiver. It is important to disable the AGC or operate at a signal level below the onset of AGC action
where the reciever's RF input and audio output are linearly related. I simply used a multimeter on AC voltage mode to
measure the receiver's audio output voltage when each antenna was connected.
I used CW mode with a narrow filter to increase the signal to noise ratio. The meter I used was just an ordinary
DMM from Radio Shack. Not fancy, but I checked its response at several different audio frequencies and amplitudes using some synthesized
tones from my computer.

Battery powered "pinger" based on a digital clock oscillator at
1843.2kHz. I used the fourth harmonic just above the 40m band at 7372.8MHz.

The test procedure involved setting up one antenna, measuring its voltage at the receiver, taking it down, and setting up the other antenna. This
substitution is somewhat time consuming if you don't think it through, but it is the
best way to get good results on a small lot. It eliminates the chances of the two
antennas under test influencing each other. It also helps you get a handle on your
random errors because you're repeatedly comparing each antenna to itself
taken down and put up again. If the antenna is especially touchy, like it has a
radiating feedline, you'll see this in the wildly varying results of one antenna
against itself. In my tests, this was not a problem. For a fixed source location,
the measured voltages on one antenna over three trials usually fell within the range
over which the voltmeter was fluctuating for any one trial. If you do these tests
and get 0.17V/0.29V/0.08V for trial one/two/three on the same antenna,
you know you're
changing something you didn't intend to change, and you can do experiments to
discover what it is. If you experiment this way, your procedure is self-correcting
and gets better and better with time. My results were more typically like
0.26V/0.26V/0.27V for Antenna 1 and 0.34V/0.34V/0.34V for Antenna 2 with perhaps
&pm;0.005V fast fluctuations as I took down and put the same antennas up three
different times. You don't
have to guess at your measurement and "uncontrolled variable" errors if you do many
trials. They will just show up in the data. You can make tiny changes to things
that should be "unimportant" and see what happens. If they're really unimportant,
nothing at all will happen.

To get an idea of how much the clutter in my yard mattered to the results, I also I
tested
the
relative signal level with the source at many locations around my yard, in proximity to different obstacles, like my nearly 30 foot mast
under my 20m Moxon. The measurement locations are depicted below.

Field strength locations with
respect to A.U.T. (Blue dot in center of radials)).

Locations 1 through 5 had the center of the 6 foot source dipole about
9 feet off the ground on a fiberglass fishing pole. Location 6 had the dipole center about 34 feet in the air. Locations 1 through 5 were
done one day, location 6 was done the next day.
Locations 1, 2 and 3 were measured closely with a tape measure so that the source support pole was 24.5 feet from the antenna.
The horizontal distance to locations 4,5, and 6 were measured using Google Earth to be 47, 40, and 72 feet respectively.
At several locations, I switched antennas several times for the sake of
checking and quantifying
the inevitable random errors. I returned to location #1 at the end
of the first day's trials to check for long-term drift.

I converted the recorded voltages to a decibel difference:

The results for twelve trials are shown below, annotated according to the aerial photo above.

Raw field strength directly taken from relative voltage measurements, annotated by measurement location.

This is already rather useful information. All the raw measurements have the ten foot antenna no worse than 2.2dB or so from a full size
quarterwave! I think that's a good deal better than telling you that I worked V55V on it. But it's not the end of the story. Unfortunately
I am not able to actually get the signal source out to a reasonable far field distance here. That means I am making near field measurements, and
the expected field strength differential will not necessarily be the same at all distances. The far field pattern of the quarter wave
and the short antenna are nearly identical, but this is not the case in the near field. The question, then, is what DO we expect?

A direct model of the field
strength test procedure to help interpret the near
field results.

We can attempt to answer that with antenna modeling. Depicted above is a model with a 50 ohm load at the feedpoint of the matched
vertical, excited
with a six foot whip at 24.5 feet out and center 9 feet up. This is a model of the actual test procedure in locations 1, 2, and 3. First I modeled
the far field gain differential of the two antennas over the same radial field. Then I modeled the voltage delivered across a 50 ohm load at
each antenna's matched feedpoint, exactly what I'm measuring at the receiver input. I did this for both antennas at each measurement location,
and developed a set of corrections to map the near field measurements to equivalent far field measurements.
In locations #1, #2, and #3, the near field measurement will make the short vertical appear 0.4dB better than it is in the far field.
So in locations 1,2, and 3, I subtracted 0.4dB from the actual measurements. At location #4, the near field measurement predicts
that the short vertical will appear 0.3dB worse than it is, so I added 0.3dB to location 4. I needed to add 0.14dB for location 5 and
location 6 had the largest error between near and far field even though it was furthest away. The model predicts that the short vertical will
appear 1.1dB worse than it is in the far field at location 6, so I added 1.1dB. The results of the near field measurements converted to equivalent
far field gain difference are shown below:

Field strength measurements
corrected to take the predicted variations into account.

The corrections based on the expected near field comparisons tighten up the measurements a bit, especially the ones taken in front of the house.
Test #5 was certainly especially down, but this location was close to the aluminum-sided house and completely in the shadow for short wavelength waves.
It shouldn't be much of a "shadow" for 7MHz waves, and I'm not going to do a detailed diffraction study around my house on 40m right now.
However, it would be interesting to move out along that heading
and see if things improve a bit further from the house. I have to go into a neighbor's yard to do that, which may be possible. I have
some interest in how obstacles like houses affect HF verticals, and Test #5 may be a good jumping off point. With the exception of Test #5 and
Test #1 as slight outliers I might plausibly claim that I have repeatable results showing that my antenna is between 0.75dB and 1.25dB down from a full size 1/4λ vertical. Pretty cool.
No bandwidth limitation, high power handling, and only 1dB down from a 30+ foot antenna using a radial system that fits in a cluttered suburban
Maryland backyard. No sanitized test range data here. I'm pretty confident that this is how it works, with all the mess of an
imperfect test lab included.

I should say that I recognize that my 1/4λ comparison antenna is
not
something that's directly very useful as a measuring stick for people
who are restricted. But this process will work for any two antennas you
want to test, and it will work if you just want to change one antenna
and see if you've improved it or made it worse. The point of these
measurements is partially to establish that my particular design is
good, but it's also to show that you can get good quantitative results
even if you
don't have more than one antenna up at once. And if you're trying to
communicate your results about a weird antenna to other people, just
pick a common comparison antenna. A hamstick-type antenna with the
exact brand and model specified might be a good comparison. Or just
report some of the "older versions" if you're just comparing the antenna
to itself. The most important thing about communicating your results is
to make sure other people could duplicate your experiments, so you need
to give people confidence that they could easily duplicate all of
your results. To this end you should always use good feedline
chokes on any antenna that needs it: balanced antennas and
elevated-radial verticals. A radiating feedline is a sure-fire way to
destroy any hope of reproducibility in your experiments.

Other Measurements: Wet Antenna and Small Transmitting Loop

One of the best parts of setting up a measurement system like this is that almost all the effort is in the initial gathering of materials
and setup. Once you get rolling it's really easy to change other variables besides the two antennas. One thing I was concerned about was the
detuning and extra loss that might come from getting the
antenna wet. As I mentioned above, I put a fiberglass rod through the coil instead of a rope because I was worried about this. But the thing is,
it's so easy to psych yourself out about problems like this because you just don't know what will happen. Certainly something will happen, but
it's hard to predict how it will change things. Depending on your personality and your mood at the time you might be paranoid about
the losses or be in denial that they're a problem. If you're already set up to measure and you want to assess the "wet antenna" though, just
grab the hose and find out what happens:

I subjected the antenna to a
"rainstorm," lost 4dB, and series base resistance skyrocketed
from 6 to 12 ohms.

I knew it could be bad, but I was frankly a bit surprised by the results. The signal immediately dropped about 4.3dB, and when I measured the base impedance without the
shunt coil the base impedance had gone up from 6 ohms to 12 ohms! If you assume a 5.4 ohm radiation resistance, that explains about 3.5dB of this
loss, and I haven't analyzed this for the mismatch loss due to the fact that the shunt coil no longer matches the new impedance to 50 ohms. If the
mismatch loss isn't bad, it is possible that a slightly lower vertical radiation resistance around 4.5 ohms explains both the wet and dry results.
The radiation resistance of verticals over dirt is a complex topic. At any rate, I dried the coil off using compressed air and the signal and
base impedance returned to their "dry" values, suggesting the losses were localized to the coil. It's not good to use lossy dielectrics in between
coil turns and tap water certainly qualifies. When you build this antenna make sure the coil has a rain shield. I put a plastic bag over it
and it rained hard all week and I had no problems. It's easy to see the problem in operation because the SWR gets quite high. Just keeping
the coil dry is at least enough to stave off most of the problem. I've
replaced the plastic bag with a cut up juice
container that I
painted
black so it looks nice.

This small transmitting loop measured about 8dB worse than the
1/4λ vertical, models suggest 4dB. Coupling to masts is probably a big source of error here.

Once you've got the test setup running and you're swapping antennas back and forth, it's really easy to
grab a few more antennas if you have them laying around and you use some care in interpreting the results. One antenna
I had laying around was my small transmitting loop, pictured above. This is
a pretty nice antenna, a four foot octagon built from 3/4 inch (0.870 inch OD) copper tubing with a Jennings UCS-300 vacuum variable and remote
motor drive. With the pinger at location #6, the furthest one out,
I took the 1/4λ and short verticals down and put the magloop up instead, tuned to resonance at 7372.8
and oriented to get maximum signal from the pinger. I
mounted it at a height that put the top of the magloop at about where the top of the short vertical was (one more military fiberglass
mast section higher than in it is in the picture). That makes the magloop and the short vertical as installed very
similar in overall physical dimensions. The magnetic loop clocked in just under 7dB worse in raw field strength compared to the small vertical, and
8dB worse than the 1/4λ vertical. However, the EZNEC prediction for that is only about 4dB worse
than a 1/4λ. There are several issues here, and some preliminary investigations of the Moxon masts' interaction
with the magloop suggest that a magloop/quarterwave A/B test is not possible in my backyard without further work, like
detuning the masts. I may be able to explain the difference with some detailed models like the investigation about
masts further down this page. The magloop coupling to masts is more complicated and different enough from the
verticals' coupling to cause problems.

The magloop still doesn't fundamentally need ground radials (though they were still there, so it was probably deriving some benefit from them) and mine
continuously covers 5-21.5MHz, no going outside to change a tap. It's a sweet little antenna. I haven't tested it at higher power but the capacitor should allow
me to run several hundred watts. But for raw performance and performance to price ratio on 40m, 30m, and 20m, my
new three meter vertical seems to be noticeably better.

Other Measurements: Influence of Masts

The field strength plots in the last section show that the relative field strength of the two antennas was not very dependent on the
measurement location, except in a way that was predictable in that region of the near field. However, the ABSOLUTE field strength at different
parts of the yard varied a fair amount, and it varied with time as well. Shown below are plots for locations 1, 2, and 3.

The field strength arbitrarily normalized to Trial #7 for both antennas.

The clump around Location 1, 0dB are trials 7, 8, and 9, and it's clear that things changed with time or a variable I wasn't
controlling from trials 1 and 2 (higher signals in location 1). I do think it's possible, however, that Location #3 was affected in absolute
field strength by the presence of my 20m Moxon mast. To investigate this a little, I modeled the two masts, grounded through 10 ohm resistors
(ground rods + bonding wires, no radials):

Far field pattern for the vertical with two parasitic masts.

It is very likely that the overall variation is even more than this, and this far field pattern would predict location 2 and location 3
to be closer than they are. But the model at least suggests an important effect, which is probably exaggerated when the test dipole is so
close to the 20m Moxon mast as it was in location 3. I would need to take a lot more data to analyze this further, but I think there's
an important lesson in it. Whatever the reason for the increased FS in trial 4 at location 3, it affected BOTH ANTENNAS about the same.
This kind of thing is to be expected to a certain extent. If you have two antennas of similar characteristics, like two verticals for the
same band, they will have a tendency to excite their surroundings in a very similar way.

It's like swapping in two different
driven elements on the same yagi. You might change things a little, especially if one is a lossy construction, but the pattern of the
yagi won't change. Neither will the pattern of my entire backyard when driven by a vertical at the test location. The pinger near the mast couples a
couple dB more strongly into the antenna under test, but it does so the same way for both antennas. This, I think, is a good example of how
the substitution A/B testing is a good technique. You hold EVERYTHING equal, including perturbing influences in the
environment, and that gives decent differential field strength results even if there's some coupling to objects that enhances or detracts
from the absolute field strength. HOWEVER, as the preliminary magnetic loop results suggest, you can't always
try
two antennas with
very different field configurations and get good answers. Two different verticals? No problem. Two different magloops
of similar physical dimensions, or identical if you're just trying different capacitors? No problem. Magloop vs.
vertical? Perhaps that's a problem.
Someday I might try to measure and choke off currents in the masts and see if I can sort this out more, but
that's a project for later.

Conclusions

So here it is. Three whole bands, just a little bit of tap switching, superb performance, easy to understand. I feel good about my solution to
the continuous tuning problem on 40m. It's simple and effective. And I didn't make something that's impossible to analyze or understand.
It's just a simple top loaded vertical. It's easy to re-design for another band, like if you want one for 160m or the enormously wide 80m band.
I see so much creative energy poured into designing electrically crazy antennas that maybe could be redirected into creatively designing
understandable antennas. Sometimes it's because people are actually playing with antennas hoping to get rich, and my message will
not get through to them. But I hope I can convince some other people that there's a lot of room for creativity within the confines of electrically
boring antennas.